In stretch blow molding plastic containers from heated preforms, individual wall portion of the preforms are drawn to different degrees resulting in wall portions of the container having different thicknesses. Drawing individual wall portions can be selectively influenced by machine parameters when heating and blowing the preforms. Accordingly, depending on the fluctuation of these parameters, the drawing varies for individual blow cavities, heating devices and/or depending on the material properties and dimensions of the preforms. For example, asymmetric wall stretching in the region of the container base can occur in relation to the outer circumference of the blown container reproducibly predetermined by the blow cavity.
To control the material distribution in the blow molded containers, it is known, for example, from U.S. 2008/0211125 A1, to reproducibly divide the wall portions of preforms by ring-shaped thickenings in the longitudinal direction and to measure the position of the thickenings on the blown container in order conclude therefrom the distribution of the wall thickness of the container thus produced. With such thickenings, however, only wall thickness distributions of the container can usually be determined in the longitudinal direction. Furthermore, the thickenings on the manufactured container are visible and—depending on the intended purpose—disturb the appearance of the container.
It is also known from DE 28 46 304 A1, within the framework of test series for optimizing the preheating of preforms for subsequent stretch blow molding, to attach grid-like markings onto the preforms and to monitor them after stretch blow molding the containers in order to estimate material distribution in the produced container wall. For ongoing production control, however, such markings have been viewed as being too expensive and not sufficiently reproducible. For example, infrared transmission measurements are used instead on the produced container walls.
Machine control of blow molding machines in dependency of results of the inspection of the produced plastic containers is also known from EP 07 93569 D1, for example, by inspection for defects in the bottle walls.
In contrast thereto, however, there is the need for methods of production control in stretch blow molding of plastic containers with which meaningful test cycles for wall thickness distribution can be realized as flexibly as possible and with little interference with ongoing production cycles.